A rotary hearth furnace is used for producing reduced iron. In the rotary hearth furnace, a pellet is prepared using coal material and metallic oxide such as iron ore and iron making dust. The reduced iron is produced by charging and heating (reducing) the pellet onto the rotary hearth which rotates in a horizontal plane in the rotary hearth furnace (refer to patent documents 1, 2, 3).
FIG. 5 is a schematic diagram showing an example of a rotary hearth furnace. In FIG. 5, reduced iron is produced by charging a pellet on a rotary hearth 21 through a pellet charge opening 22 and heating (reducing) the pellet. The rotary hearth 21 rotates in the horizontal plane in a rotary hearth furnace 20. The reduced iron is moved toward the circumference of the rotary hearth 21 by a screw conveyer 23 and then discharged out of the furnace through a discharge opening 24. The screw conveyer generally has a water-cooled structure provided in a rotary shaft. A screw blade is made of material having heat resistance and abrasion resistance.
On the other hand, there is a technique for reducing attrition of a screw blade, which reduces an apparent weight of the screw conveyer using an elevating cylinder to set a pressing force applied to a rotary hearth within a predetermined range (larger than or equal to 4000 N/m and smaller than or equal to 20000 N/m) (refer to Patent Document 4).
[Patent Document 1] Japanese Patent Publication No. 45-19569
[Patent Document 2] Japanese Patent No. 3020482
[Patent Document 3] U.S. Pat. No. 4,636,127
[Patent Document 4] Japanese Patent Publication No. 2005-61651
In the rotary hearth furnace described above, the screw blade is used at high temperature. In addition, the screw blade constantly receives frictional force since the screw blade contacts a surface of the rotary hearth when scraping out the reduced iron lying on the rotary hearth. Therefore, in the conventional structure, the screw blade is worn for a short period due to the attrition and can not be continuously used for a long period. In this case, it is necessary to often bring out the screw conveyer from the rotary hearth furnace and check the screw conveyer. Thus an operation rate of the rotary hearth furnace is reduced.
In an apparatus disclosed in Patent Document 4, the pressing force of the screw conveyer applied to the rotary hearth is set within the predetermined range to reduce the attrition of the screw blade.
However, in the apparatus disclosed in Patent Document 4, it is necessary to adjust the force of the elevating cylinder for pushing up the screw conveyer so that the pressing force of the screw conveyer falls within the predetermined range. When the force for pushing up the screw conveyer is wrongly adjusted, the apparent weight of the screw conveyer may be excessively reduced. In addition, if the force of the elevating cylinder for pushing up the screw conveyer is reduced using a spring and soon, the number of components is increased and the apparatus increases in cost.